Considerable research activity has recently been focused on the interaction between superconductivity and long-range magnetic order. See, for example, M. B. Maple, in the Proceedings of the 15th International Conference on Low Temperature Physics, Journal de Physique Supplement C6-1374 (1978), and M. Ishikawa, O. Fischer and J. Muller, ibid. C6-1379 (1978). S. Maekawa and M. Tachiki, Physical Review B 18, 4688 (1978). H. Suhl, Journal of Less Common Metals 62, 225 (1978). E. I. Blount and C. M. Varma, Physical Review Letters 42, 1079 (1979). Two classes of ternary compounds exhibiting these phenomena have been reported: the Chevrel phases typified by RE.sub.x Mo.sub.6 S.sub.8 (O. Fischer, A. Treyvaud, R. Chevrel, and M. Sergent, Solid State Communications 17, 721 (1975)) and RE.sub.x Mo.sub.6 Se.sub.8 (R. N. Shelton, R. W. McCallum and H. Adrian, Physics Letters A 56, 213 (1976) and the tetraborides such as (RE)Rh.sub.4 B.sub.4. B. T. Matthias, E. Corenzwit, J. M. Vandenberg, and H. Barz, Proceedings of the National Academy of Sciences, U.S.A. 74, 1334 (1977).